WHERE: Rocky Narrows Reservation, Sherborn, Mass. Meet at parking lot at corner of E. Goulding and Forest Streets near north end of reservation.

A team of University of Massachusetts Amherst scientists led by entomologist Joseph Elkinton will release approximately 2,000 parasitic flies at the Trustees of Reservations’ Rocky Narrows Reservation in Sherborn, Mass., on Friday morning, May 9, to control caterpillars of winter moth, an invasive species in eastern Massachusetts.

Winter moth caterpillars are now stripping foliage from many kinds of deciduous trees from Mystic, Conn. to coastal Maine, Elkinton says. Both the moth and its natural enemy, the parasitic fly known as Cyzenis albicans, are originally from Europe. The fly has successfully controlled moth invasions in Nova Scotia and the Pacific Northwest.

Flies lay their eggs on foliage where the caterpillars eat them. They hatch inside the caterpillar and develop into a larval fly. Winter moth caterpillars drop to the ground in late May to form pupae in the soil. Those with larval flies inside soon die and an adult fly emerges the following spring to attack more winter moth caterpillars. “I’m confident that this fly will eventually suppress winter moth populations to harmless levels,” says Elkinton. “That is what happened in Nova Scotia, where the fly was introduced in the 1950s, and where winter moths have been at low levels ever since.”

He and his colleagues have been releasing the fly at seven sites in Massachusetts, Maine and Rhode Island for the past several years. In Wellesley, more than 30 percent of the larvae have been killed by the fly over the past two years and moth densities have declined dramatically. The entomologist says it will take a few years for the few thousand released flies to reproduce enough to catch up with millions of winter moths. In Nova Scotia it took six years.

Researchers are confident that the fly will not cause other problems. It attacks only winter moth, not other caterpillars, and they do not bother humans. Flies to be released in Sherborn were collected in British Columbia and shipped to a USDA quarantine facility at Otis Air Base on Cape Cod to overwinter. The Cyzenis release project represents a cooperative effort between the University of Massachusetts Amherst, the USDA Forest Service, USDA Animal and Plant Health Inspection Service and the Massachusetts Department of Conservation and Recreation.